Foldable display neutral axis management with thin, high modulus layers

ABSTRACT

A foldable display of a computing device includes a back stiffening layer, a transparent frontplate layer, a transparent cover window layer, and an OLED display layer disposed between the back stiffening layer and the transparent frontplate layer. The OLED display layer characterized by a Young&#39;s modulus that is lower than the Young&#39;s modulus of the transparent frontplate layer and that is lower than the Young&#39;s modulus of the back stiffening layer; a neutral plane of the foldable display is located within the OLED display layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Nonprovisional of, and claims priority to, U.S.Patent Application No. 62/517,137, filed on Jun. 8, 2017, entitled“FOLDABLE DISPLAY NEUTRAL AXIS MANAGEMENT BY ADDING THIN, HIGH MODULUSLAYERS”, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

This description relates to thin foldable displays and, in particular,to thin film foldable displays in which the neutral axis of the displayis precisely controlled to reduce mechanical stress on the display.

BACKGROUND

Modern computing devices often attempt to achieve a balance betweenportability and functionality. A tension can exist between having adisplay that provides for a rich display of information on a singlesurface, which suggests a relatively large form factor of the device toaccommodate a relatively large display, and a device that is smallenough to be easily carried and accessed by a user, which suggests arelatively small form factor of the device.

A potential solution to address this dilemma is to use a foldableflexible display in the computing device, so that in the display'sfolded configuration, the computing device has a relatively small formfactor, and in the display's unfolded configuration, the computingdevice can have a relatively large display. To keep the form factor ofthe computing device small and slim, it is desirable to have relativelythin displays. However, folding a relatively thin display can result insmall radius bends at the fold in the display, which may be detrimentalto sensitive components of the display, for example, thin filmtransistors (TFTs), organic light-emitting diodes (OLEDs), thin-filmencapsulation (TFE) and the like. In addition, thin displays can berelatively fragile and in need of protection against breakage fromimpacts to the front surface of the device.

It can be difficult to create foldable top-emitting plastic OLEDdisplays that have a small folding radius in both directions (i.e.,having two surfaces of the display fold both towards each other and awayfrom each other) and that can survive many fold-unfold cycles. Inparticular, creating sturdy, durable Z-fold displays (i.e., displayswith both inward and outward folds) is greatly complicated by thefragility of the thin-film layers in the display stack.

One approach is to building the stack of layers for a functional displayis to use optically clear adhesive (OCA) to join different functionallayers of the stack. For example, a display stack may include from thefollowing layers:

-   -   1. Backplate layer    -   2. Adhesive layer    -   3. Display layer (including polyimide substrate with barrier,        TFT, OLED, and encapsulation layers)    -   4. OCA layer    -   5. Touch sensitive layer (typically a multi-layer film stack)    -   6. OCA layer    -   7. Polarization layer (including a circular polarizer)    -   8. OCA layer    -   9. Cover Window (CW) layer (user-facing cover window film)

In some implementations, the polarization layer and the touch sensitivelayer may be reversed, combined or eliminated. A common developmentdirection involves building touch functionality directly on top of thedisplay layer. This reduces the thickness of the stack of the mostfragile layers and also simplifies electrical connection to the touchlayer. In such implementations, the stack may include the followinglayers:

-   -   1. Backplate layer    -   2. Adhesive layer    -   3. Display-Touch layer    -   4. OCA layer    -   5. Polarization layer (including a circular polarizer)    -   6. OCA layer    -   7. CW layer (user-facing cover window film)

In another implementation, the cover window layer and the polarizationlayer can be combined, so that the stack includes the following layers:

-   -   1. Backplate layer    -   2. Adhesive layer    -   3. Display-Touch layer    -   4. OCA layer    -   5. POL-CW layer

The Display-Touch layer often is manufactured in a very expensive,highly-automated OLED factory using a highly optimized recipe thatcannot easily be altered to meet customer customer-specificrequirements. The backplate and polarization/cover window layers may becustomer-specific and are typically added in a less expensive factorysetting after the display exits the OLED line. However, thesecustomer-specific backplate layer and polarization/cover window layersmay cause the neutral plane of the device to shift away from thedisplay-touch layer, which may be detrimental to the in-system foldingcycle life of the entire display.

Thus, foldable display devices in which the neutral plane of the deviceis in, or close to, the most fragile display layers, are desirable.

SUMMARY

In one general aspect, a foldable display of a computing device includesa back stiffening layer, a transparent frontplate layer, a transparentcover window layer, and an OLED display layer disposed between the backstiffening layer and the transparent frontplate layer. The OLED displaylayer characterized by a Young's modulus that is lower than the Young'smodulus of the transparent frontplate layer and that is lower than theYoung's modulus of the back stiffening layer; a neutral plane of thefoldable display is located within the OLED display layer.

Implementations can include one or more of the following features,alone, or in any combination with each other. For example, thetransparent front plate can include glass fibers and polymer materials.A touch layer can be disposed between the back stiffening layer and thetransparent frontplate layer. The OLED display layer and the touch layercan be fabricated as a single layer. There can be no layers between theback stiffening layer and the single layer. There can be no layersbetween the transparent frontplate and the single layer.

The OLED display layer can be configured to be bent repeatedly to aradius of less than 10 mm. A neutral plane of the foldable display canbe located within a middle 50% of the OLED display layer. A neutralplane of the foldable display can be located within a middle 20% of theOLED display layer. An optically clear adhesive layer can be locatedbetween the OLED display layer and the transparent frontplate layer. Thefoldable display can be configured to be folded at a first location in afirst direction and can be configured to be folded at a second locationin a second direction that is opposite to the first direction.

In another general aspect, a computing device can include memoryconfigured for storing executable instructions, a processor configuredfor executing the instructions, and a foldable display configured fordisplaying information in response to the execution of the instructions.The foldable display can include: a back stiffening layer, a transparentfrontplate layer a transparent cover window layer, and an OLED displaylayer disposed between the back stiffening layer and the transparentfrontplate layer. The OLED display layer can be characterized by aYoung's modulus that is lower than the Young's modulus of thetransparent frontplate layer and that is lower than the Young's modulusof the back stiffening layer, wherein a neutral plane of the foldabledisplay is located within the OLED display layer. The computing devicealso can include a bend limit layer arranged substantially parallel tothe OLED display layer, with the bend limit layer being configured toincrease its stiffness non-linearly when a radius of a bend of the bendlimit layer is less than a threshold radius of curvature of the foldabledisplay layer, the threshold radius of curvature being greater than 1 mmand less than 20 mm.

Implementations can include one or more of the following features,alone, or in any combination with each other. For example, a couplinglayer can be disposed between the bend limit layer and the OLED displaylayer, with the coupling layer having a Young's modulus lower than theYoung's modulus of the OLED display layer. The bend limit layer caninclude a material having a coefficient of thermal expansion within 50%of the coefficient of thermal expansion of the OLED display layer. Thebend limit layer can include a material having a coefficient of thermalexpansion within 25% of the coefficient of thermal expansion of the OLEDdisplay layer. An overall thickness of the foldable display is less thanone millimeter.

The computing device can also include a touch layer disposed between theback stiffening layer and the transparent frontplate layer. The OLEDdisplay layer and the touch layer can be fabricated as a single layer. Aneutral plane of the foldable display can be located within a middle 50%of the OLED display layer. A neutral plane of the foldable display islocated within a middle 20% of the OLED display layer. The computingdevice can include an optically clear adhesive layer between the OLEDdisplay layer and the transparent frontplate layer. The foldable displaycan be configured to be folded at a first location in a first directionand can be configured to be folded at a second location in a seconddirection that is opposite to the first direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a computing device that includes afoldable display with a single inward fold and the foldable display in apartially folded configuration.

FIG. 2 is a perspective view of the computing device with a singleinward fold, with the display in a folded configuration.

FIG. 3 is a schematic diagram of a flexible display device having aplurality of bendable sections that are bendable in differentdirections.

FIG. 4 is a schematic diagram of a flexible display device having astack of a number of different layers.

FIG. 5 is a schematic diagram of a foldable display having a bendablesection that is bent around a minimum radius, R_(min).

FIG. 6 is a graph showing an example stiffness curve for a foldabledisplay in which the limit radius is reached when the foldable displayis folded.

FIG. 7 is a schematic diagram of a foldable display having a bendablesection that is bent around a minimum radius, R_(min).

FIG. 8A is a schematic diagram of an example implementation of a bendlimit layer.

FIG. 8B is a top view of a thin film showing a plurality of bondingsites at which the film is bonded to the base portions of differentadjacent segments.

FIG. 9 is a schematic diagram of a plurality of adjacent segments foruse in a bend limit film.

FIG. 10 is a schematic diagram of a rotating mold that can be used in anexample molding process for forming the adjacent segments.

FIG. 11 is a schematic diagram of a mold that can be used for formingadjacent segments of a bend limit layer.

FIG. 12 is a schematic diagram of another implementation of the foldabledisplay, in which a bend limit layer is coupled to a display layer.

FIG. 13 is a schematic diagram of the foldable display when it is in abent configuration.

FIG. 14 is a schematic diagram of another implementation of a display inwhich a bend limit layer is coupled to a display layer.

FIG. 15 is a schematic diagram of the foldable display when it is in abent configuration.

FIG. 16 is a schematic diagram of another implementation of a foldabledisplay in which a bend limit layer is coupled to a display layer.

FIG. 17 is a schematic diagram of a foldable display when the display isin a bent configuration.

FIG. 18 is a schematic diagram of another implementation of a foldabledisplay in which a bend limit layer is coupled to a display layer.

FIG. 19 is a schematic diagram of the foldable display when it is in abent configuration.

FIGS. 20A, 20B, 20C, 20D are schematic diagrams of details of thefoldable display of FIGS. 18 and 19.

FIGS. 21A, 21B, 21C, and 21D are schematic diagrams of details of thefoldable display of FIGS. 18 and 19.

FIG. 22 is a schematic diagram of a foldable display having a bendablesection that is bent around a minimum radius, R_(min).

DETAILED DESCRIPTION

As described herein, to control the location of the neutral plane in thefinal display device, after fabrication of the display-touch layer, athin back stiffening layer and a thin transparent frontplate layer, bothhaving high modulus, can be laminated with thin bondlines or depositedon either side of the display-touch layer. By sandwiching the delicatedisplay-touch layers between two stiff outer layers, the location of theneutral plane can be stabilized, and subsequent layers that are added oneither side can have less influence on the neutral axis location, thusimproving in-system reliability. In implementations, the back stiffeninglayer can be combined with the backplate layer to create asurface-stiffened backplate layer.

FIG. 1 is a perspective view of a computing device 100 that includes afoldable display with a single inward fold and the foldable display 102in a partially folded configuration. The device 100 has the foldabledisplay 102 mounted so that it folds with the viewable face inward. Itis also possible to mount the foldable display 102 on the opposite sideof device 100 so that the display folds with viewable face outward (notshown). FIG. 2 is a perspective view of the computing device 100, withthe display 102 in a folded configuration. The foldable display 102 maybe, for example, a TFT (Thin-Film-Transistor) OLED (Organic LightEmitting Diode) display, or other appropriate display technology. Thefoldable display 102 may comprise appropriate circuitry for driving thedisplay to present graphical and other information to a user.

As shown in FIG. 1 and FIG. 2, the foldable display 102 can include afirst relatively flat rigid, or-semi-rigid, section 112, a second flatrigid section 114, and a third bendable section 116. In someimplementations, the foldable display 102 can include more than two flatrigid sections 112, 114 and more than one bendable section 116. In someimplementations, the foldable display 102 can include zero, or only one,flat rigid section 112, 114. For example, when a foldable display 102includes zero flat rigid sections, the display 102 can be continuouslybendable, and can be rolled up, as in a scroll. The foldable display 102shown in FIG. 1 and FIG. 2 has a bendable section 116 that allows thefoldable display to bend about an axis. In other implementations, thefoldable display 102 can include bendable sections that allow the bladeto bend about more than one axis.

The bendable section 116 of the foldable display 102 allows the display102 to bend in an arc that has a radius, and the bendable section can bemade to become rigid when the radius of the bendable section reaches aspecified minimum radius. This minimum radius may be selected to preventthe display from bending in a radius so small that fragile components ofthe display would be broken. In some implementations, the minimum radiusis greater than or equal to 2.5 millimeters, or greater than or equal to3.0 millimeters, or greater than or equal to 5 millimeters. Thus, thebendable section can be flexible when bent in a radius greater than theminimum radius and then become rigid when the bend radius is equal to orsmaller than the minimum radius.

FIG. 3 is a schematic diagram of a flexible display device 300 having aplurality of bendable sections 304, 306 that are bendable in differentdirections. The flexible display device 300 can have a display surface302 a, 302 b, 302 c that can take on a “Z” shaped when the device isfolded in its folded, compact configuration, with a portion of thedisplay surface 302 a, 302 b folded inward with the surfaces 302 a, 302b facing each other, and a portion of the display 302 c folded outward.To assume the “Z” shaped configuration, the display device 300 can behave a bendable section 304 that is bendable in a clockwise direction,as shown in FIG. 3, and a bendable section 306 that is bendable in acounter-clockwise direction, as shown in FIG. 3.

FIG. 4 is a schematic diagram of a flexible display device 400 having astack of a number of different layers. For example, in someimplementations, a flexible organic light-emitting diode (OLED) layer410 can be sandwiched between a back stiffening layer 414 and atransparent frontplate layer 406. The OLED layer 410 can include, atleast, OLED functionality to generate the visual information displayedby the display device 400. In some implementations, the OLED layer 410can also include touch-sensitive elements to detect a user's touch atparticular locations on the display device 400 and to generateelectrical signals in response to the detected touch, and in someimplementations, a layer (not shown in FIG. 4) separate from the OLEDlayer 410 can include the touch-sensitive elements Furthermore, in someimplementations, the OLED layer 410 can include other functionalelements of the display device 400, such as, for example, TFTs, andencapsulation layer, and anti-reflection optical elements to reduceglare from the display, but in some implementations these functionalelements can be included in separate layers from the OLED layer. In someimplementations, the OLED layer 410 can be coupled to the frontplatelayer 406 by an optically clear adhesive (OCA) layer 408. An opticallyclear adhesive layer 404 can be applied to a front surface of thefrontplate layer 406 to couple the frontplate layer 406 to a coverwindow film layer 402 that serves to protect the device on the frontside. In general, different adjacent discrete layers of the device 400can be joined by an adhesive material between the adjacent materials.Adhesive material between an optical path from the OLED emitters of theOLED layer 410 and user's eye are OCA.

In some implementations, the OLED layer 410 can be coupled to the backstiffening layer 414 by an adhesive layer 412. In some implementations,the OLED layer 410 can be directly deposited on the back stiffeninglayer 414. In some implementations, the back stiffening layer 414 can becoupled to a backplate layer 418, for example, by an adhesive layer 416,or can be directly bonded to the backplate layer 418. In someimplementations, the back stiffening layer 414 can be combined with thebackplate layer 418 to form an integrated a surface-stiffened backplatelayer. As explained in more detail below, the mechanical properties ofthe back stiffening layer 414 and the frontplate layer 406 can becontrolled to manage the location of the neutral axis of a finishedproduct that incorporates the display device 400.

Because the thickness of each layer of the stack is important to theoverall thickness of the device 400, it is desirable to have arelatively thin thickness for the layers. For example, in somenon-limiting examples, the thickness of the flexible OLED layer 410 canbe on the order of approximately 50 μm; the thickness of frontplatelayer 406 and the back stiffening layer 414 can be on the order ofapproximately 50 μm; the thickness of the optically clear adhesivelayers 404, 408, 412 can be on the order of approximately 25 μm; thethickness of the cover window layer 402 can be on the order ofapproximately 100 μm; and the thickness of the backplate layer can be onthe order of approximately 25 μm. Thus, an overall thickness of thedevice 400 can be on the order of a millimeter and the device can havelayers with individual thicknesses that are fractions of a millimeter.In some implementations, the overall thickness of the display device 400can be less than one millimeter.

The components of the stack of the device 400 have differentas-fabricated properties, including stresses and strains that exist inthe components when the layer is fabricated. Additional stresses andstrains can be induced in the layers of the stack when the display isbent into a configuration that is different from the configuration inwhich the layer was fabricated. For example, if the layer was flat whenit was fabricated, then additional strain can be induced by stretchingor bending the layer, and if the layer was fabricated in a curvedconfiguration, then additional strain can be induced by flattening thelayer. If the bend-induced strain exceeds a threshold valuecharacteristic of a component of the stack, the component can be damagedby the strain due to cracking, buckling, delamination, etc. Thischaracteristic damage threshold strain may be different depending ontemperature, humidity, required cycle life, and other use andenvironmental factors. Brittle inorganic layers of the stack cantypically withstand less strain than inorganic layers before they aredamaged by the strain. Nevertheless, organic materials in the stack alsocan be damaged by excessive strain that is induced by bending.

FIG. 5 is a schematic diagram of a foldable display 500 having abendable section 501 (the curved portion shown in FIG. 5) that is bentaround a minimum radius, R_(min). The foldable display 500 can includean OLED layer 502 that includes components that generate images on thedisplay (emitted from the side of the display that faces toward theinside of the bend), a high-modulus back stiffening layer 504, ahigh-modulus frontplate layer 512, and a cover window-polarization layer514. The frontplate layer can be coupled to the OLED layer 502 and tothe cover window-polarization layer 514 with OCA. The back stiffeninglayer 504 can be coupled to the OLED layer with an adhesive, which doesnot need to be OCA. The modulus of the layers 502, 504, 512 can beparameterized by the Young's modulus of the layer. The modulus of theback stiffening layer 504 and the frontplane layer 512 can be greaterthan then modulus of the OLED layer 502. The display 500 can alsoinclude a bend limit layer 520 that limits the radius at which thefoldable display 400 can bend to greater than or equal to the minimumradius,

When the OLED layer 502 is fabricated in a flat configuration, thenbending the OLED layer 502 in the absence of the bend limit layer 520may cause the bendable section to assume a radius less than the minimumradius, R_(min), which may induce excessive strain within the OLED layer502. The OLED layer 502 can be characterized by a plane 506 at which nostrain is induced when the OLED layer 502 is bent. This plane isreferred to herein as the “neutral plane” 506. When the OLED layer 502is bent and the neutral plane is in the middle of the OLED layer 502,compressive strain may be induced along the inner radius of the bend,R_(inner), and tensile strain will be induced along the outer radius ofthe bend, R_(outer).

If the stack of materials and material thicknesses within the device 500is symmetrical about a midplane of the OLED layer 502, then the neutralplane 506 corresponds to the midplane of the layer 502. However,different material properties (e.g., thickness, Young's modulus, etc.)of different layers within the device 500 can cause the neutral plane506 to be displaced above or below the midplane of the OLED layer 502.For example, having a thick, high-modulus layer on only one side of theOLED layer 502 will move the neutral plane toward the high-moduluslayer. The location of the neutral plane within the device 500, alongwith the maximum tolerable strain values of the materials within thelayers of the device 500, determines the minimum bend radius that can betolerated without causing damage to components within the device 500,especially fragile components in the OLED layer 502.

The bend limit layer 520 can be attached to the OLED layer 502 toprovide support for the OLED layer 502 and also can prevent the OLEDlayer 502 from being bent around a radius that is smaller than itsminimum tolerable bend radius. In some implementations, thefunctionality of the bend limit layer 520 can be combined in a singlelayer with the functionality of the back stiffening layer 504. The bendlimit layer 520 can be reinforced with materials (e.g., reinforced withhigh-strength fibers) to provide strength and support for the device.Materials in bend limit layer can have a coefficient of thermalexpansion (CTE) that is close to the CTE of the OLED layer 502, so thatthe fragile components are not unduly stressed by thermal cycling of thedevice 500. For example, while many fiber materials have CTE's that areclose to zero or even negative, some ceramic fibers can have CTE's onthe order of 8 ppm per Kelvin. Use of such fiber materials can improvethermal expansion matching to a wide range of structures, including OLEDdisplay layers. In some implementations, the CTE of the fibers can bewithin about 50% of the CTE of the OLED display layer 502. In someimplementations, the CTE of the fibers can be within about 25% of theCTE of the OLED display layer 502. In some implementations, the CTE ofthe fibers can be within about 10% of the CTE of the OLED display layer502.

The bend limit layer 520 can be relatively flexible when it bent inradii such that the radius of the inner portion of the OLED layer 502 isgreater than R_(min) and then can become stiff and inflexible when theradius of the bend approaches, or matches, R_(min). Stiffness can beparameterized by the change in bend radius per unit of applied forcethat causes the foldable display 500 to bend. For example, in FIG. 6,when the display is folded in half around a 180 degree bend, twice theradius of the bend is shown by the parameter, x, when a force, F, isapplied to bend the foldable display. The stiffness of the foldabledisplay 500 then can be parameterized by the derivative, k=dF/dx. Thestrength of the foldable display can be characterized as the maximumforce, F, that the foldable display 500 can withstand before failure ofthe display occurs.

When the foldable display 500 is laid flat in its folded configuration,it can be maintained in its folded configuration by the force of gravityon the upper folded portion of the display, such that zero additionalforce is needed to be applied to the upper folded portion to maintainthe foldable display in its flat folded configuration, or, in otherimplementations, additional force can be applied by external means suchas latches, magnets, etc. to maintain the display in its foldedconfiguration. In this configuration the radius of the bend can bedefined as the limit radius, R_(limit), i.e., the radius at which theback stiffening layer 504 limits the further bending of the foldabledisplay unless additional external force is applied. To bend thefoldable display further from this configuration requires additionalexternal force to overcome the stiffness of the bend limit layer 520.Thus, an example stiffness curve for a foldable display in which thelimit radius is reached with the foldable display is folded 180 degrees,showing stiffness as a function of x is shown in FIG. 5.

It can be advantageous to have a foldable display with a stiffness curvethat exhibits a relatively sharp increase in stiffness once the limitradius is reached, such that the foldable display can be easily foldedinto its folded configuration in which R_(limit) is close to R_(min),and then the foldable display will become quite stiff, such that itremains in this configuration despite forces pressing it toward a radiussmaller than R_(limit).

The bend limit layer 520 is shown on the outside of the bend in FIG. 5,with the OLED-display layer 502 being disposed in the stack toward theinside of the bend. However, the bend limit layer 520 also can be on theinside of the bend 501, for example, as shown in FIG. 7, in which caseOLED layer 502 is on the outside of the bend and the content displayedby the display is on the outside of the bend 501.

In some implementations (particularly when the OLED layer 502, the CWlayer 514 and the frontplate layer 512 are on the outside of the bend501), glass used in one or more of the layers 502, 512, 514 can befabricated to avoid the glass forming sharp shards when the glass isbroken. For example, in one implementation, the glass used in one ormore of the layers 502, 512, 514 can be treated with patternedion-implantation (e.g., a grid pattern), so that when the glass breaksit is more likely to break along, or between, the pattern, thus avoidingsharp shards of glass.

The mechanical properties of the back stiffening layer 504, and thefrontplate layer 512 can be controlled, so as to maintain the neutralplane 506 at, or close to the mid-plane of the fragile OLED layer 502,so that the OLED layer 502 can tolerate relatively small bend radii.Because other layers of the stack (e.g., the bend limit layer 520, theCW-polarization layer 514, etc.) can affect the location of the neutralplane 506 within the device 500, the mechanical properties (e.g., thethicknesses, densities, material composition, etc.) of the backstiffening layer 504 and the frontplate layer 512 must be selected inrelation to those of other layers in the stack to maintain the neutralplane at or near the midplane of the OLED layer 506. In someimplementations, the mechanical properties of the back stiffening layer504, and the frontplate layer 512 can be controlled, so as to maintainthe neutral plane 506 within the OLED layer. In some implementations,the mechanical properties of the back stiffening layer 504, and thefrontplate layer 512 can be controlled, so as to maintain the neutralplane 506 within the middle 50% of the OLED layer. In someimplementations, the mechanical properties of the back stiffening layer504, and the frontplate layer 512 can be controlled, so as to maintainthe neutral plane 506 within the middle 20% of the OLED layer.

Referring again to FIG. 4, the back stiffening layer 414 or thesurface-stiffened backplate layer that includes the properties of theback stiffening layer 414 can be opaque since light from the OLED layer410 does not need to be transmitted through it. Therefore, the backstiffening layer 414 can be made using a large variety of materials andprocesses. However, the frontplate layer 406 must be transparent,because light from the OLED layer 410 must be transmitted through it.Ordinary plastic films are ill-suited as materials for the frontplatelayer 406, because their modulus is relatively low, and transparentoxide thin films can be too fragile. However, the frontplate layer 406can be made from high-modulus, transparent composites, such as, forexample, the glass-fiber and polymer materials disclosed Jin, J., et al.“Rollable Transparent Glass-Fabric Reinforced Composite Substrate forFlexible Devices,” Adv. Mater. 22(40), 4510-4515 (2010), which isincorporated herein by reference. Other high-modulus, transparentmaterials, such as, for example, a thin glass layer (e.g., about 30μm-50 μm thick), which may include high quality soda-lime or which mayinclude ion-exchange strengthened alumino-silicate.

Because the frontplate layer 406 can be covered and protected by theCW-polarization layer 402, delicate materials of the transparentfrontplate layer 406 that rely on being clean and defect-free to achievethe desired mechanical properties of the frontplate layer 406 can beprotected during system assembly and end use. To additionally reducesurface damage and breakage during frontplate layer lamination, theglass can be supplied in roll format with a thin, adhesion-enhancing andprotective polymer layer already applied on each side.

FIG. 8A is a schematic diagram of an example implementation of a bendlimit layer 800. The bend limit layer 800 can include a plurality ofadjacent segments 802 that are each separated from neighboring segmentsfor R>R_(limit) and that are in contact with neighboring segments whenR≤R_(limit). Each segment 802 can have a base portion 804 that isattached to a thin film 806 and a head portion 808 that is wider in adirection parallel to the plane of the bend limit layer 800 than thebase portion 804. For example, the thin film 806 can be bent in radii ofless than 3 mm. The thin film 806 has a thickness that is small comparedwith the height of the segments 806 in a direction perpendicular to thethin film 806. The stiffness of the thin film 806 is low, so that thebend limit layer 800 is easily bent for radii R≥R_(limit). The thin film806 can be bent in radii small enough to accommodate the designparameters of the bend limit layer 800. In one non-limiting example, thethin film 806 can have a thickness of about 50 μm and when bent into aradius of 2.5 mm can experience a 1% strain. Of course, the thickness ofthe material can be adjusted to trade off advantages between differentparameters, for example, the minimum radius of the thin film can be bentinto, the strength of the thin film, and the stiffness of the thin film.

One example material that could be used is the polyimide film known asKapton® HN available from DuPont in thicknesses of 7.6 μm, 12.7 μm, 25.4μm 50.8 μm, etc. Another example material that could be used is a thinmetal foil. For example, a 12 μm thick stainless steel foil has a strainof about 0.3% when bend into a radius of 2 mm.

In the example implementation shown in FIG. 8A, the bond line betweenthe base portions 808 and the thin film 806 is covered by 50% of onesurface of the thin film 806. In other words, half of the surface of thethin film 806 is attached to base portions 804 of adjacent segments 802,and half of the surface is unattached. Other configurations are alsopossible, in which the bond line coverage is more or less than 50%. Theportion of the thin film 806 that is bonded to the adjacent segments 802is much stiffer than the portions that are not bonded. This increasesthe stain in the unbonded portions of thin film 806, and this increasemust be accounted for in the materials and geometry of the bend limitlayer 800. With the head portions 808 being wider than the base portions804, such that less than 100% of the thin film 806 is covered by thebase portions 808, the portions of the thin film 806 between the baseportions can flex and bend to allow the bend limit layer 800 to be bentto a small radius.

In some implementations, the base portions 804 of the adjacent segments802 are not bonded to the thin film 806 continuously in a direction intothe page, as shown in FIG. 8A, and base portions 804 of adjacentsegments 802 are not bonded to the thin film 806 at locations shown inthe cross section of FIG. 8A. Rather, bonding sites between the baseportions 804 of adjacent segments 802 and the thin film 806 can beoffset in the direction into the page, as shown in FIG. 8A.

FIG. 8B is a top view of the thin film showing a plurality of bondingsites at which the film 806 is bonded to the base portions 804 ofdifferent adjacent segments 802. For example, a first group 830 ofbonding sites 832, 834, 836 can bond the film 806 to a first segment,whose footprint on the film 806 is shown by rectangle 830, and a secondgroup 840 of bonding sites 842, 844, 846 can bond the film 806 to asecond first segment, whose footprint on the film 806 is shown byrectangle 840. Because the bonding sites of adjacent segments are offsetfrom each other along the direction 850, portions of the film 806between adjacent segments that are not directly bonded to the segmentscan flex relatively easily when the bend limit film is bent into a smallradius, as shown in FIG. 8A. For example, the film between bonding sites832 and 862 that is under the second segment, whose footprint on thefilm 806 is shown by rectangle 840, can flex when the bend limit film isbent, even if the footprints 830 and 840 butt up against each other.

The head portion 808 of each segment 802 can have vertical sides 810that, when the bend limit film 806 is flat, are not perfectlyperpendicular to the thin film 806, but rather that are angled towardeach other as they extend away from the thin film 806. Then, when thebend limit layer 800 is bent into a radius equal to R_(limit), thevertical sides 810 of adjacent segments 802 become in intimate contactwith, and parallel to, each other, so that they form a rigid, ruggedlayer of material that has a high stiffness for R≤R_(limit). Some meansof fabricating the head portion 808 of each segment 802 may not haveperfectly flat sides, but instead have other surface geometries thatalso allow both faces of adjacent segments 802 come in intimate contactwith each other, so that they form a rigid, rugged layer of materialthat has a high stiffness for R≤R_(limit).

The segments 802 can be formed from a number of different materialsincluding, for example, metals, polymers, glasses, and ceramics.Individual blocks can be molded, machined, drawn (e.g., through a shapedwire) and then attached to the thin film 806 at the correct spacing. Inanother implementation, a plurality of adjacent segments 802 can beformed simultaneously and then attached to the thin film 806.

For example, as shown in FIG. 9, a plurality of adjacent segments 802can be formed on a substrate 804, for example, by a single- ormulti-step molding process, and then, after the segments 802 are bondedto the thin film 806, the substrate 804 can be broken, dissolved, orotherwise removed from the segments 802. In another implementation, theplurality of adjacent segments 802 can be formed on a substrate 804, forexample, by a lithography and etching process, and then, after thesegments 802 are bonded to the thin film 806, the substrate 804 can bebroken, dissolved, or otherwise removed from the segments 802.

FIG. 10 is a schematic diagram of a rotating mold that can be used in anexample molding process for forming the adjacent segments 802. Forexample, slides 1, 2, 3, etc. can be inserted radially into positionwith respect to a core pin, and then material can be injected into thevoids between the slides and the core pin to simultaneously form thesegments 802 and the thin film 806. As segments 802 are formed, theassembly can be rotated counter-clockwise and the slides can be removedin numerical order to free segments 802 from the counter-clockwise-mostposition in FIG. 9 while new segments are formed in positions clockwisefrom the counter-clockwise-most position. By using transparent toolingand an ultra-violet (UV) rapid-curing molding compound, high productionthroughput can be achieved.

FIG. 11 is a schematic diagram of a mold 1102 that can be used forforming adjacent segments 802 of a bend limit layer 800. The shape ofthe mold 1102 can correspond to the shape of the bend limit layer 800,when the bend limit layer is in its designed limit radius configuration.Then, the adjacent segments 802 of the bend limit layer 800 can beformed as a unified part within the mold 1102, however, withimperfections along the designed boundaries 1104 between adjacentsegments 802. The imperfections then can allow the unified part to becracked along the designed boundaries between the adjacent segments, sothat after the bend limit layer 800 is removed from the mold 1102 andflattened the bend limit layer 800 has the separated adjacent segments802 shown in FIG. 8A, but when the bend limit layer 800 is bent to itslimit radius, the adjacent segments form strong, rugged contacts totheir adjacent segments.

FIG. 12 is a schematic diagram of another implementation of the foldabledisplay 1200, in which a bend limit layer 1202 is coupled to a displaylayer 1204. The bend limit layers 1202 can include a plurality ofsublayers. The sublayers can include, for example an outer layer 1206, amiddle layer 1208, and an inner layer 1210. The inner layer 1210 caninclude one or more fingers 1212 that extends outward toward the outerlayer 1206 and that, when the bend limit layer 1202 is in a relaxed,unbent configuration, are each horizontally separated by a gap 1214 inthe plane of the layers from a portion of the middle layer 1208 that isclosest to the middle of the bend into which the bend limit layer 1202can be bent. Two fingers 1212 and gaps 1214 are shown in FIG. 12, butany number of fingers and corresponding gaps is possible.

The layers can be made of different materials. In one implementation theinner and outer layers 1210, 1206 can be made of an easily deformable,low stiffness metal, such as a nickel titanium alloy (e.g., Nitinol),and the middle layer can be made of a stiffer metal, such as stainlesssteel. The middle layer can be thicker than the inner and outer layers.

FIG. 13 is a schematic diagram of the foldable display 1200 when it isin a bent configuration. As shown in FIG. 13, compressive strain on theinner layer at the apex of the bend due to the bending of the foldabledisplay causes the gaps 1214 between the fingers 1212 of the inner layerand the middle layer to be closed. Thus, the sections of the inner layer1210 can act as leaves that move across the inner layer in response tothe compressive strain and that pull their corresponding fingers withthem. When the gaps 1214 are closed, the stiffness of the bend limitlayer 1202 increases, so that further bending of the foldable display isrestricted.

FIG. 14 is a schematic diagram of another implementation of the display1400 in which a bend limit layer 1402 is coupled to a display layer1404. The bend limit layers 1402 can include a plurality of sublayers.The sublayers can include, for example, an outer skin layer 1406, amiddle layer 1408, and an inner skin layer 1410. The layers can be madeof different materials. In one implementation, the inner and outerlayers 1410, 1406 can be made of very thin layer of a material with veryhigh elongation (e.g. Nitinol film), and the middle layer can be made ofmaterial whose stiffness changes as a function of the bend radius of thefoldable display 1400.

FIG. 15 is a schematic diagram of the foldable display 1400 when it isin a bent configuration. As shown in FIG. 14, compressive strain on theinner layer 1408 due to the bending of the foldable display causes thestiffness of the middle layer 1408 to increase. This can occur in anumber of different ways. In one implementation, the compressive strainon the inner layer 1410 and the middle layer 1408 causes the layers1410, 1408 to deform inward toward the center of the bend, and thedeformation of the material can increase the stiffness of the materialsin the layers. In another implementation, the compressive strain on theinner layer 1410 and the middle layer 1408 causes a changes of state ofan electromechanical device (e.g., a piezoelectric device) 1412 withinat least one of the layers 1410, 1408.

In some implementations, electro-active material can be used in middlelayer 1408, where the stiffness of the electro-active material canchange in response to a voltage or current that is applied to thematerial. The electro-active material can include, for example, (1)ferroelectric-based materials (e.g., polyvinylidene fluoride-basedferroelectric citric polymer materials), (2) ionic-based material (e.g.,ionomeric polymer-metal composites (e,g, Nafion or Flemion) orelectroactive polymer gels), (3) non-ionic based materials, (e.g., polyvinyl alcohol-based materials); (4) carbon nanotube or conductiveparticles embedded in a polymer matrix, and (5) conductive polymer basedmaterials (e.g., Polypyrrole, Polyaniline, Polythiophene, Polyacetelene,Poly-p-phenylene, Poly-phenylene vinylene. In some implementations, theelectro-active material can change its rheological properties (such asstorage modulus and/or loss modules) upon the application of an electricfield. In some cases, the storage modulus of the material can be changedby more than 3 orders of magnitude by applying an electric field of afew kilovolts per millimeter to the material. In some cases, the formfactor of the electro-active material can be changed upon theapplication of a voltage to bend, twist, expand, contract or shrink thematerial. The electric field and/or current can be applied to theelectro active material by a dedicated electrode in the stack of thedisplay device or by one or more electronic elements present in otherstructures of the device (e.g., electrodes in a touch layer of thedevice).

In some implementations, the compressive strain on the inner layer 1410and the middle layer 1408 can cause a changes of state of anelectromechanical device (e.g., a piezoelectric device) 1412 within atleast one of the layers 1410, 1408, and a signal due to the change ofstate can be used to cause a change in the stiffness of the middle layer1408. For example, an electrical signal from the electromechanicaldevice 1412 in response to the bend-induced strain can trigger anelectrical current or a voltage to be applied to the materials in themiddle layer, which, in turn, can cause a change of state and stiffnessof the material in the middle layer. For example, the material canchange from a liquid to a solid in response to the applied current orvoltage, or material can be pumped into the bent portion of the middlelayer, or the orientation of particles of material can be rearranged inresponse to the applied current or voltage to increase the stiffness ofthe bent portion. Other modalities of changes to the stiffness of theelectro-active material in response to an applied electric field orcurrent are also possible.

FIG. 16 is a schematic diagram of another implementation of the foldabledisplay 1600 in which a bend limit layer 1602 is coupled to a displaylayer 1604. The content of the display can be displayed on a surface ofthe display that is on the opposite side of the foldable display 1600from the bend limit layer 1602 (e.g., facing down, as shown in FIG. 16).The bend limit layer 1602 can include a plurality of threads or fibersarranged across the layer 1602 in a plane and that, when the bend limitlayer 1602 is in a flat configuration, are arranged in a serpentineconfiguration, so that the length of each fibers is longer than thestraight end-to-end distance in the plane between the ends of eachfiber. The fibers can be made of strong, low-stretch material, such as,for example, fibers made from glass, Kevlar®, graphite, carbon fiber,ceramics, etc. and can be laid down in a low modulus substrate. Forexample, the fibers can be laid down via a spread tow technique in thedesired pattern using specialized manufacturing equipment. The fiberscan be pinned at locations 1606 along their lengths to a layer of thefoldable display, e.g., to a substrate in the bend limit layer 1602 orto an surface at interface between the bend limit layer 1602 and thedisplay layer 1604. For example, the fibers can be pinned at nodes ofthe serpentine configuration of the fibers. The pinning can be performedby a number of different techniques. For example, a laser heatingprocess may bond the fibers at the pinning sites to the layer, or thefibers can be mechanically bonded at the sites.

The fibers can limit the bend radius of the foldable display 1600 whenthe display is bent, when the bend limit layer 1602 is on the outside ofthe bend and the display layer 1604 is on the inside of the bend,because the fibers can become straight and limit the bend radius of thefoldable display when the desired minimum bend radius is reached. Inother words, the resistance of the bend limit layer 1602 to tensilestrain in the layer is very low while the fibers are unstretched andthen becomes high when the fibers are stretched to their full lengths.With the fibers bonded to material in the bend limit layer 1602 at thepinning sites, a sudden increase in stiffness of the bend limit layeroccurs when the bending of the bend limit layer 1602 causes the fibersto become straight between adjacent pinning sites 1606.

FIG. 17 is a schematic diagram of a foldable display 1600 when thedisplay is in a bent configuration with the bend limit layer 1602 on theoutside of the bend and with the display layer 1604 on the inside of thebend. In this configuration, when the bend limit layer is under tensilestrain, the fibers can be become straight in the curved plane of thebend limit layer 1602, and the end-to-end distance, within the curvedplane, of each fiber segment between adjacent pinning sites 1606 can beclose to, or the same as, the length of each fiber between the adjacentpinning sites 1606. In this configuration the strong, low-stretch fibersresist the tensile strain on the bend limit layer, and thereby limit thebend radius of the foldable display 1600.

FIG. 18 is a schematic diagram of a foldable display device with a bendlimit layer having a patterned structure of materials that can have anon-linear stiffness response to compressive forces caused by bending ofthe bend limit layer.

In one implementation, the patterned structure can include an array ofribs 1806 that extend away from the display layer 1804. As shown in FIG.18, the ribs 1806 can be rectangular shaped, but other shapes are alsopossible. The ribs 1806 can be relatively rigid, in that they have ahigh bulk modulus and a high shear modulus. Therefore, the ribs 1806retain their shape when the foldable display 1800 is bent. The ribs caninclude a variety of different rigid materials, including, for example,metals (e.g., aluminum, copper, steel, etc.) ceramic materials, glassmaterials, etc.

Gaps or trenches 1808 between adjacent ribs 1806 can be partially orfully filled with a second material that has a non-linear stiffnessresponse to compressive forces caused by bending of the foldable display1800. The material can include a foam (e.g., and open cell foam), a gel,or other material whose bulk modulus changes as a function of thecompressive forces on the material.

When the bend limit layer 1802 is in a relaxed, unbent configuration, asshown in FIG. 18, the material in the gaps 1808 between the ribs 1806can have a low bulk modulus and a low stiffness. For example, in therelaxed unbent configuration, the gaps 1808 can be filled with thenon-linear stiffness material. The distance between adjacent ribs at thedistal ends of the ribs (i.e., away from the display layer 1804) can beapproximately equal to the distance between adjacent ribs 1806 at theproximate ends of the ribs (i.e. closest to the display layer 1804).

FIG. 19 is a schematic diagram of the foldable display 1800 when it isin a bent configuration. As shown in FIG. 19, compressive strain at theinterface of the display layer 1804 and the bend limit layer 1802 layercan cause the distance between adjacent ribs 1806 at the proximate endsof the ribs to be less than when the bend limit layer 1802 is in itsrelaxed, unbent configuration. In addition, because of the bend of thebend limit layer 1802 and the non-zero length of the ribs the distancebetween adjacent ribs at the distal ends of the ribs 1806 is evenshorter when the bend limit layer 1802 is in the bent configuration thanwhen in the unbent configuration. Consequently, the material in the ingaps or trenches 1808 between the ribs 1806 is squeezed when the layer1802 is bent. The squeezing of the material can cause a sudden increasein the stiffness of the material when the radius of the bend becomesless than a threshold radius. For example, in the case of an open cellfoam material in the gaps 1808 between the ribs 1806, air can besqueezed of the cells when the material is compressed, and when acritical amount of air has been squeezed from the material when theradius reaches the threshold radius, then the stiffness of the materialcan suddenly increase.

Although rectangular ribs 1806 are illustrated in FIGS. 18 and 19, andrectangular gaps 1808 between the ribs 1806 are shown in FIG. 18, othershapes of both the ribs and the material in the gaps between the ribsare possible. For example, as shown in FIG. 20A, ribs 2002 can begenerally T-shaped profile. In another example, as shown in FIG. 20B,ribs 2004 can have a generally trapezoid-shaped profile. In anotherexample, as shown in FIG. 20C, ribs 2006 can have a profile that isnarrower in the middle than at the top and the bottom of the ribs. Inanother example, as shown in FIG. 20D, ribs 2008 can have a customshaped profile that is configured, in conjunction with the type andshape of the material in the gaps between the ribs to accomplish adesired stiffness vs. bend radius response.

Correspondingly, the shape of the materials in the gaps between theribs, which materials have a non-linear stiffness response to the radiusof curvature of the bend limit film, can have different shapes. Forexample, FIGS. 21A, 21B, 21C, and 21D show rectangular gaps betweenrectangular ribs 2102, but with the materials in the gaps havingdifferent shapes in the different figures. For example, as shown in FIG.21A, the rectangular gaps can be filled with non-linear stiffnessresponse material 2104 that bulges above the tops of the gaps when thebend limit layer is in its relaxed configuration. In another example, asshown in FIG. 21A, the rectangular gaps can be filled with non-linearstiffness response material 2106 that precisely fills the rectangulargaps when the bend limit layer is in its relaxed configuration. Inanother example, as shown in FIG. 21C, the rectangular gaps can befilled with non-linear stiffness response material 2108 that descendsbelow the tops of the gaps when the bend limit layer is in its relaxedconfiguration. In another example, as shown in FIG. 21D, the rectangulargaps can be filled with non-linear stiffness response material 2110along the sides and bottom of the gaps, but on in the central portion ofthe gaps. The type and shape of the material in the gaps between theribs can be selected to accomplish a desired stiffness response to thebend radius response of the bend limit layer.

FIG. 22 is a schematic diagram of a foldable display 2200 having abendable section 2201 that is bent around a minimum radius, R_(min). Thefoldable display 2200 can include an OLED layer 2202 that includescomponents (e.g., OLED layers, TFT layers, touch screen layers, etc.)that generate images on the foldable display and a bend limit layer 2204that limits the radius at which the foldable display 2200 can bend togreater than or equal to the minimum radius, R_(min). The display 2200also includes a high-modulus back stiffening layer 2212 and ahigh-modulus frontplate layer 2214. The bend limit layer 2204 can becoupled to the OLED layer 2202 by a coupling layer 2203. The couplinglayer 2203 can include, for example, an adhesive material or a bondingmaterial on respective surfaces that touch the OLED layer 2202 and thebend limit layer 2204.

As described above, when the OLED layer 2202 is fabricated in a flatconfiguration, bending the OLED layer 2202 induces compressive strainalong the inner radius of the bend, and tensile strain is induced alongthe outer radius of the bend. It is desirable to keep the neutral plane2206 of the assembly, at which no stain occurs in response to thebending, at, or close to, the plane in which fragile and sensitivecomponents of the assembly (e.g., TFTs) exist. Thus, the coupling layer2203 can include low modulus material (e.g., rubber, gel, etc.), so thatlittle strain within the planes of the layers is transmitted between theOLED layer 2202 and the bend limit layer 2204. The display 2200 caninclude a high-modulus back stiffening layer 2212 and/or a high-modulusfrontplate layer 2214 on the opposite sides of the OLED layer 2202 thatfunction to maintain the neutral plane close to its designed locationwithin the OLED layer 2202 when the bend limit layer 2204 acts to limitthe bend radius of the display 2200. For example, the layers 2212, 2214can have stiffnesses compensate for the effect of the stiffness of thebend limit layer on the position of the neutral plane, so that theneutral plane does not shift from its designed location in the OLEDlayer 2202 when the OLED layer 2202 is coupled to the bend limit layer2204.

The devices and apparatuses described herein can be included as part ofa computing device, that includes, for example, a processor forexecuting instructions and a memory for storing the executableinstructions. Specific structural and functional details disclosedherein are merely representative for purposes of describing exampleembodiments. Example embodiments, however, be embodied in many alternateforms and should not be construed as limited to only the embodiments setforth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term and/or includes any and all combinations of one ormore of the associated listed items.

It will be understood that when an element is referred to as beingconnected or coupled to another element, it can be directly connected orcoupled to the other element or intervening elements may be present. Incontrast, when an element is referred to as being directly connected ordirectly coupled to another element, there are no intervening elementspresent. Other words used to describe the relationship between elementsshould be interpreted in a like fashion (e.g., between versus directlybetween, adjacent versus directly adjacent, etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms a, an and the areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the termscomprises, comprising, includes and/or including, when used herein,specify the presence of stated features, integers, steps, operations,elements and/or components, but do not preclude the presence or additionof one or more other features, integers, steps, operations, elements,components and/or groups thereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedconcurrently or may sometimes be executed in the reverse order,depending upon the functionality/acts involved.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, e.g., those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise, or as is apparent from the discussion,terms such as processing or computing or calculating or determining ofdisplaying or the like, refer to the action and processes of a computersystem, or similar electronic computing device, that manipulates andtransforms data represented as physical, electronic quantities withinthe computer system's registers and memories into other data similarlyrepresented as physical quantities within the computer system memoriesor registers or other such information storage, transmission or displaydevices.

Lastly, it should also be noted that whilst the accompanying claims setout particular combinations of features described herein, the scope ofthe present disclosure is not limited to the particular combinationshereafter claimed, but instead extends to encompass any combination offeatures or embodiments herein disclosed irrespective of whether or notthat particular combination has been specifically enumerated in theaccompanying claims at this time.

What is claimed is:
 1. A foldable display of a computing device, the foldable display comprising: a back stiffening layer; a transparent frontplate layer; a transparent cover window layer; and an OLED display layer disposed between the back stiffening layer and the transparent frontplate layer, the OLED display layer characterized by a Young's modulus that is lower than the Young's modulus of the transparent frontplate layer and that is lower than the Young's modulus of the back stiffening layer, wherein a neutral plane of the foldable display is located within the OLED display layer.
 2. The foldable display of claim 1, wherein the transparent frontplate layer includes glass fibers and polymer materials.
 3. The foldable display of claim 1, further comprising a touch layer disposed between the back stiffening layer and the transparent frontplate layer.
 4. The foldable display of claim 3, wherein the OLED display layer and the touch layer are fabricated as a single layer.
 5. The foldable display of claim 4, wherein there are no layers between the back stiffening layer and the single layer.
 6. The foldable display of claim 4, wherein there are no layers between the transparent frontplate and the single layer.
 7. The foldable display of claim 1, wherein the OLED display layer is configured to be bent repeatedly to a radius of less than 10 mm.
 8. The foldable display of claim 1, wherein a neutral plane of the foldable display is located within a middle 50% of the OLED display layer.
 9. The foldable display of claim 1, wherein a neutral plane of the foldable display is located within a middle 20% of the OLED display layer.
 10. The foldable display or claim 1, further comprising an optically clear adhesive layer between the OLED display layer and the transparent frontplate layer.
 11. The foldable display of claim 1, wherein the foldable display is configured to be folded at a first location in a first direction and is configured to be folded at a second location in a second direction that is opposite to the first direction.
 12. A computing device comprising: memory configured for storing executable instructions; a processor configured for executing the instructions; a foldable display configured for displaying information in response to the execution of the instructions, the foldable display including: a back stiffening layer; a transparent frontplate layer; a transparent cover window layer; and an OLED display layer disposed between the back stiffening layer and the transparent frontplate layer, the OLED display layer characterized by a Young's modulus that is lower than the Young's modulus of the transparent frontplate layer and that is lower than the Young's modulus of the back stiffening layer, wherein a neutral plane of the foldable display is located within the OLED display layer; and a bend limit layer arranged substantially parallel to the OLED display layer, the bend limit layer configured to increase its stiffness non-linearly when a radius of a bend of the bend limit layer is less than a threshold radius of curvature of the foldable display, the threshold radius of curvature being greater than 1 mm and less than 20 mm.
 13. The computing device of claim 12, further comprising a coupling layer disposed between the bend limit layer and the OLED display layer, the coupling layer having a Young's modulus lower than the Young's modulus of the OLED display layer.
 14. The computing device of claim 12, wherein the bend limit layer includes a material having a coefficient of thermal expansion within 50% of the coefficient of thermal expansion of the OLED display layer.
 15. The computing device of claim 12, wherein the bend limit layer includes a material having a coefficient of thermal expansion within 25% of the coefficient of thermal expansion of the OLED display layer.
 16. The computing device of claim 12, wherein an overall thickness of the foldable display is less than one millimeter.
 17. The computing device of claim 12, further comprising a touch layer disposed between the back stiffening layer and the transparent frontplate layer.
 18. The computing device of claim 17, wherein the OLED display layer and the touch layer are fabricated as a single layer.
 19. The computing device of claim 12, wherein a neutral plane of the foldable display is located within a middle 50% of the OLED display layer.
 20. The computing device of claim 12, wherein a neutral plane of the foldable display is located within a middle 20% of the OLED display layer.
 21. The computing device of claim 12, further comprising an optically clear adhesive layer between the OLED display layer and the transparent frontplate layer.
 22. The computing device of claim 12, wherein the foldable display is configured to be folded at a first location in a first direction and is configured to be folded at a second location in a second direction that is opposite to the first direction. 